1. Field of the Invention
The invention relates to palletizers and, more specifically, relates to a palletizer having an improved slide plate assembly and an indexing pallet hoist. The invention additionally relates to an improved method of indexing a pallet hoist of a palletizer.
2. Background of the Invention
Palletizers are well known for automatically or semi-automatically stacking layers of articles such as bags or cartons on a pallet, sheet, or other support. The typical palletizer includes two vertically-spaced conveyor assemblies which convey articles and supports, respectively.
The upper conveyer assembly typically includes a slide plate assembly (sometimes known as a stripper plate assembly) onto which articles are conveyed in a pattern suitable for depositing as a layer onto a stack. The upper conveyor assembly may also include an upstream slat sorter, a turner assembly, and an accumulator conveyor assembly that interact to arrange the conveyed articles into the desired pattern. The layer is typically transferred from the accumulator conveyor assembly to the slide plate assembly by a rake assembly or the like.
The lower conveyor assembly includes a vertically-movable hoist (sometimes known as an indexing pallet hoist) that is positioned beneath the slide plate assembly, that is raiseable to receive layers of articles one at a time from the slide plate assembly to form a stack, and that indexes downwardly to receive each successive layer. The lower conveyor assembly also may include an outfeed conveyor for conveying a completed stack of articles away from the hoist as well as a staging mechanism and other equipment for conveying a stack of pallets, sheets, or other supports toward the hoist and for delivering the supports one at a time to the hoist.
The slide plates of the typical palletizer deposit a layer of articles onto the underlying hoist by initially supporting the layer on one or more imperforate metal plates and by driving the plate(s) to slide out from under the layer to deposit the layer onto the underlying stack. Some employ a single slide plate moving away from the outfeed end of the rake assembly. Others employ two facing plates that move away from one another to define so-called "bi-parting stripper plates." A palletizer employing a slide plate assembly of this type is manufactured by HK Systems, Inc. under the name von Gal as Model No. VGHS4000.
Traditional slide plate assemblies employing imperforate longitudinally moving slide plates exhibit drawbacks when handling relatively high-friction articles such as plastic bags. The articles tend to stick to the metal plates, particularly during warm weather, and bunch and roll up so that they do not form an even layer of uniform dimensions. The imperfectly-formed layer is poorly suited for stacking on previously-formed layers and also is poorly suited for forming a base for subsequent layers. The problem of sticking and rolling is exacerbated by two features of the traditional slide plate assembly design.
First, the imperforate metal surface of the slide plate has a relatively high frictional coefficient. Contact between the bag or other article and the slide plate hinders smooth sliding motion of the article across the plate. The article also tends to stick to the moving plate during a stripping or depositing operation.
Second, the aligned orientation of the slide plate(s) relative to the direction of conveyance of the upper conveyor assembly requires relatively lengthy movement between the slide plate(s) and the articles both during movement of articles onto the slide plate assembly and during stripping motion of the slide plates. This lengthy movement results from the fact that the conveyed articles and the stripper plate(s) move in the same direction so that the articles must travel a substantial distance before reaching their final position above the hoist. The plate area that must be traversed by a layer of articles prior to its final positioning is commonly known in the art as "deadplate." Minimizing deadplate would minimize the area of frictional contact between the layer of articles and the slide plate(s).
Another problem associated with the traditional palletizer resides in the operation of its indexing pallet hoist. In some applications, and particularly in the palletizing of bags or other articles which may become partially filled with air or otherwise may not be as flat as otherwise would be desired for stacking, the hoist is used to compress each layer of articles against the bottom surface of the slide plate assembly to force the air out of the articles and to flatten them. That is, after a layer of articles is deposited onto the hoist (or on another layer of articles if another layer of articles has been previously deposited), the hoist is lowered or indexed downwardly to provide clearance for the slide plates, the slide plates are closed, and then the hoist is raised or indexed upwardly to force the upper surface of the upper layer of articles into compressive engagement with the bottom surface of the slide plates. The hoist is then lowered again or indexed downwardly to receive another layer.
Hoist raising and lowering is typically achieved using a variable-speed electric motor. The amount of torque imposed by the motor must be controlled in order to impose the desired compressive force on the upper layer of articles. Traditional palletizers control motor torque using a current sensing relay that cuts off power supply to the electric motor upon sensing a rise in current occurring due to article engagement with the bottom surface of the slide plate assembly. A disadvantage of this approach is that the motor is necessarily overloaded during each compressing operation. Premature wear and failure of the motor and other components and/or article bursting may result. Moreover, it is very hard to control or set the motor of traditional systems for different article properties.